Advent of Code problem 1.1 (Common Lisp)

problem1.1.cl

; I'm a new lisper, this code is probably gross :)

; misc resources:
; https://hyperpolyglot.org/lisp
; http://www.lispworks.com/documentation/HyperSpec/Body/f_parse_.htm
; https://gigamonkeys.com/book/files-and-file-io.html
; https://lispcookbook.github.io/cl-cookbook/iteration.html
; http://www.ai.sri.com/pkarp/loop.html

; read a file as a list of lines.
; thanks to https://stackoverflow.com/a/3814098
(defun read-lines (filename)
    (with-open-file (stream filename)
        (loop for line = (read-line stream nil)
            while line
            collect line)))

; convert a list of strings into a list of integers.
(defun as-ints (strs)
    (mapcar (function parse-integer) strs))

; (princ (as-ints (read-lines "problem1.input.txt")))

(defparameter *previous* nil)
(defparameter *current* nil)
(defparameter *depth-increase-count* 0)

(loop for depth in (as-ints (read-lines "problem1.input.txt"))
    do (cond ((not *current*) (setq *current* depth))
             (t (progn (setq *previous* *current*)
                       (setq *current* depth)
                       (if (> *current* *previous*) (setq *depth-increase-count* (+ 1 *depth-increase-count*)))))))

(print *depth-increase-count*)

Here's a follow-up version which doesn't use global variables:

; misc resources:
; https://hyperpolyglot.org/lisp
; http://www.lispworks.com/documentation/HyperSpec/Body/f_parse_.htm
; https://gigamonkeys.com/book/files-and-file-io.html
; https://lispcookbook.github.io/cl-cookbook/iteration.html
; http://www.ai.sri.com/pkarp/loop.html

; read a file as a list of lines.
; thanks to https://stackoverflow.com/a/3814098
(defun read-lines (filename)
    (with-open-file (stream filename)
        (loop for line = (read-line stream nil)
            while line
            collect line)))

; convert a list of strings into a list of integers.
(defun as-ints (strs)
    (mapcar (function parse-integer) strs))

; return the number of increases in the list of depths.
(defun count-depth-increases (depths)
    (let ((current nil) (previous nil) (total 0))
        (loop
            for depth in depths
            do (if (not current) (setq current depth))
                   (progn (setq previous current)
                          (setq current depth)
                          (if (> current previous) (incf total))))
        total))

; main:
(let* ((lines (read-lines "problem1.input.txt"))
       (depths (as-ints lines))
       (num-depth-increases (count-depth-increases depths)))
    (print num-depth-increases))